Optical data storage disks have gained widespread acceptance for the storage, distribution and retrieval of large volumes of information. Optical data storage disks include, for example, audio CD (compact disc), CD-R (CD-recordable), CD-RW (CD-rewritable) CD-ROM (CD-read only memory), DVD (digital versatile disk or digital video disk), DVD-RAM (DVD-random access memory), HD-DVD (high definition-DVD), Blu-ray, and various other types of writable or rewriteable media, such as magneto-optical (MO) disks, phase change optical disks, and others. Some newer formats for optical data storage disks are progressing toward smaller disk sizes and increased data storage density. For example, some new media formats boast reduced track pitch, increased storage through multiple data layers and increased storage density using blue-wavelength lasers for data readout and/or data recording.
Optical data storage disks are typically produced by first making a data storage disk master that has a surface pattern that represents encoded data and/or precision tracking features on the master surface. The surface pattern, for instance, may be a collection of precisely dimensioned grooves or other features that define master pits and master lands, e.g., typically arranged in either a spiral or concentric manner. The master is typically not suitable as a mass replication surface, as the master features are typically defined within an etched photoresist layer formed over a master substrate.
After creating a suitable master, that master can be used to make a stamper, which is less fragile than the master. The stamper is typically formed of electroplated metal or a hard plastic material, and has a surface pattern that is the inverse of the surface pattern encoded on the master. An injection mold can use the stamper to fabricate large quantities of replica disks. Also, photopolymer replication processes, such as rolling bead processes, have been used to fabricate replica disks using stampers. In any case, each replica disk may contain the data and tracking precision that was originally encoded on the master surface and preserved in the stamper. The replica disks can be coated with a reflective layer and/or a phase change layer, and are often sealed with an additional protective layer. Additional stampers (later generation stampers) can also be made from the first generation stamper, to improve productivity with respect to one original master, or to allow for master features to be formed as the inverse of the desired replica disk features.
Some types of optical data storage disks contain multiple substrates bonded together. For example, DVD and HD-DVD media constructions are composed of two substrates, both with the thickness of approximately 0.6 millimeters. Typically, one of the substrates bears the data and/or tracking information, while the other non-information substrate provides the functions of backside protection, reference clamping area, a labeling area, and improved mechanical stability. In these types of bonded media constructions, the information-bearing surface is sandwiched interior to the two substrate bond. As such, the information-bearing surface is interrogated by a drive system using a beam of laser light brought to focus through the information substrate. This, in turn, places strict optical requirements for thickness, thickness uniformity, index of refraction, and birefringence for the information-bearing substrate. The non-information substrate provides backside protection inasmuch as the information-bearing surface is encased between the two bonded substrate elements. The backside of the medium refers to the non-information substrate side of the media which is not interrogated by the drive system laser. The media clamping area refers to the radial portion of the region located between the inner perimeter of the medium and the information area containing the data layers which is designated by the drive specification for a drive clamping reference zone. The clamping area of the media may be used by the drive system to hold the media in fixed relative position to the drive spindle. The non-information substrate also functions to improve robustness of the medium by improving both the dynamic and static mechanical stability. A bonded non-information substrate improves static mechanical stability by minimizing bending stress to the information-bearing surface. Likewise, a properly bonded non-information substrate improves dynamic mechanical runout for the rotating media.
Blu-ray media constructions may likewise be viewed as being composed of two substrate elements, one having a thickness of approximately 1.1 millimeters and another with a thickness of approximately 0.1 millimeters. In this view, the substrate element of greater thickness is fabricated with an information-bearing surface including data and/or precision tracking information and the substrate of lesser thickness covers the information-bearing surface to provide protection and defocusing of surface contamination. This substrate of lesser thickness is often referred to as a cover layer. In this media construction, the thicker substrate element provides the functionality of information bearing as well as providing functions of backside protection, clamping area and improved mechanical stability. The beam of laser light interrogates the information-bearing surface for this media construction through the cover layer. This, in turn, places strict optical requirements such as thickness, thickness uniformity, index of refraction, and birefringence on the cover layer.
Different types of optical data storage media may also vary based on the distance between the data and the objective lens of the reading mechanism. With DVD and HD-DVD, the data layer is located approximately 0.6 millimeters in from the surface of the disk closest to the laser. With Blu-ray, the data layer is located approximately 0.1 millimeters in from the surface of the disk closest to the laser. During reading and recording, the laser beam passes through the portion of the disk located between the laser and the data layer. This portion of the disk, the optically functional portion, must adhere to specified requirements in regard to optical thickness, optical thickness uniformity, allowable thickness range, allowable birefringence, and range of optical index of refraction for the material.
Air incident optical data storage disks may contain a data layer that is located on the surface of the disk closest to the laser. Since this type of medium does not have additional disk layers located between the data layer and the laser, the distance between the data and the objective lens of the reading mechanism can be very short. Also, since the laser beam does not pass through any additional disk layers prior to the data layer, a large portion of these disks may be non-optically functional. The non-optically functional portion of the disk, defined as the portion that the laser beam does not pass through, however, can still provide backside protection, clamping area, and improved mechanical stability.